Ultrafine powders have significant potential for a wide range of applications including catalysts, magnetic recording media, optoelectronic materials, magnetic fluids and composite materials. Ultrafine metallic powders have been prepared by physical methods, such as vapour deposition and sputtering, which have high quality, i.e. clean surfaces and uniform particle size distribution. However, industrial applications for such powders are limited by low yield rates and high cost. Alternative chemical production methods, such as thermal decomposition and precipitation are currently being studied for the preparation of a wide range of powders. Chemical methods can provide large quantities of ceramic powders for industrial applications. However, except for precious metals, chemical methods are generally not applied to the production of metallic powders.
Mechanical activation has been used for the production of fine powders with particle sizes typically in the range of 0.2 to 2 microns. One method for the production of powders by mechanical activation is the process of mechanical alloying described in U.S. Pat. No. 3,591,362, by which alloys are formed from pure starting materials by milling a mixture of the powders in a high energy ball mill. During milling the constituent particles undergo repeated collisions with the grinding balls causing deformation, welding and fracture of the particles which result in microstructural refinement and composition changes leading to the formation of nanocrystalline or amorphous alloys.
Another example of the use of mechanical activation to form fine powders, as described in U.S. Pat. No. 5,328,501, is concerned with a mechanochemical reduction process. This process involves the mechanically activated chemical reduction of reducible metal compounds with a reductant during milling in a high energy ball mill, to refine and manufacture metals, alloys and composite powders. During milling the energy imparted to the reactants through ball/reactant collision events causes repeated welding and fracture of the reactant particles. Consequently oxidation/reduction reactions occur at welded interfaces and reaction kinetics are enhanced without the need for high temperatures or melting to increase intrinsic reaction rates.
With both the above described prior art mechanically activated processes micron sized particles are formed which contain a nanometre scale mixture of phases, crystallites or amorphous regions. These processes have not previously been known to result in powders containing a significant fraction of particles with sizes less than 50 nm or interconnected networks of ultrafine particles exhibiting high values of specific surface area.